This invention relates to a method for providing a coating on the surface of a polymeric substrate. More particularly, it relates to an improved method for providing an abrasion-resistant, radiation-cured coating on the surface of a polycarbonate substrate, and the article produced from the process.
Polymeric substrates are often provided with protective coatings. When it is important that the substrate be transparent, it is generally also important that the coating exhibit high optical quality while also retaining its other physical attributes, such as abrasion resistance and high gloss.
The coatings of the prior art are typically formed from various synthetic polymers and contain considerable amounts of volatile organic solvents which enhance the flow and leveling characteristics of the coating on the substrate. However, the inclusion of considerable amounts of volatile organic solvents creates several problems. First, the volatile material has to be eliminated from the cured coating so that it will not decrease the integrity of the coating. Therefore, expensive dryer systems have to be implemented to evaporate the solvents after the film is applied. Furthermore, other expensive equipment is required to remove the organic vapors from the work facility, and to comply with environmental requirements regarding atmospheric emissions and waste water disposal. Moreover, the presence of organic solvents creates-explosion and fire hazards which have to be eliminated through the use of additional safety equipment and explosion-proof facilities. Finally, the purchase of solvents as additional raw materials for the coatings represents a substantial expense which further increases the cost of the resulting products.
One method for reducing the presence of organic solvents in coatings of this nature involves the use of radiation-curable coatings formed from various monomers and cross-linking oligomers, in which the monomer acts as a solvent by providing the necessary physical and theological properties for the uncured coating. Curing occurs when radiation is applied directly to the article surface having the coating thereon. The monomer enters the polymerization with the cross-linking oligomers and does not volatilize. The need for volatile solvents is thus eliminated or at least greatly reduced. However, several disadvantages exist with radiation-curable coatings. For instance, polymerizable monomers are not as effective as volatile solvent systems in the enhancement of flow and leveling of the coating on the substrate and therefore, the optical quality of the finished article is compromised. Moreover, oxygen which is present in the vicinity of the coating (and, to a lesser extent, within the coating material itself) often must be excluded during the curing process so that a coating having high optical clarity, abrasion resistance and chemical resistance may be formed. The elimination of oxygen is accomplished by continuously purging the cure chamber with nitrogen gas. However, a significant expense is required for the purchase of large volumes of nitrogen and for the maintenance of the purging equipment. Furthermore, safety equipment must be installed to protect operating personnel from accidental asphyxiation.